1. Field of the Invention
The present invention relates to methods and apparatus for making polyamide filaments, such as nylon 6,6, having high tensile strength at high spinning speeds. The invention also relates to yarns and other articles formed from such filaments.
2. Related Prior Art
Many synthetic polymeric filaments, such as polyamides, are melt-spun, i.e., they are extruded from a heated polymeric melt. Melt-spun polymeric filaments are produced by extruding a molten polymer through a spinneret with a plurality of capillaries. The filaments exit the spinneret and are then cooled in a quench zone. The details of the quenching and subsequent solidification of the molten polymer can have a significant effect on the quality of the spun filaments.
Methods of quenching include cross-flow, radial, and pneumatic quench. Cross-flow quenching is frequently used for producing high strength polyamide fibers and involves blowing cooling gas transversely across and from one side of the freshly extruded filamentary array. In cross-flow quenching, airflow is generally directed at a right angle to the direction of movement of the freshly extruded filaments.
In radial quench, the cooling gas is directed inwards through a quench screen system that surrounds the freshly extruded filamentary array. Such cooling gas normally leaves the quenching system by passing down with the filaments and out of the quenching apparatus.
Both cross-flow quench and radial quench are limited to fiber production at relatively low speed, about 2,800-3,000 meters per minute, for high tenacity application. Higher production speeds increase the number of broken filaments during the draw stages. Broken filaments interrupt the process continuity and decrease the product yield.
In the 1980's, Vassilatos and Sze made significant improvements in the high-speed spinning of polymeric filaments, especially polyester filaments. These improvements are disclosed in U.S. Pat. Nos. 4,687,610, 4,691,003, and 5,034,182.
These patents disclose gas management techniques, whereby gas surrounds freshly extruded filaments to control their temperature and attenuation profiles. These types of quench systems and methods are known as pneumatic quench or pneumatic spinning systems. Other pneumatic quenching methods include those described in U.S. Pat. No. 5,976,431 and U.S. Pat. No. 5,824,248.
The pneumatic quench spinning process provides an advantage of reduced filament and, subsequently, reduced yarn tension during spinning. In general this reduced yarn tension provides better productivity via higher spinning speeds with reduced filament breaks and a processability advantage for the wound yarn. Generally, pneumatic quenching involves supplying a given volume of cooling gas to cool a polymeric filament. Any gas may be used as a cooling medium. The cooling gas is preferably air, because air is readily available. Other gases may be used, for instance steam or an inert gas, such as nitrogen, if required because of the sensitive nature of the polymeric filaments, especially when hot and freshly extruded.
In pneumatic spinning, the cooling gas and filaments travel substantially co-linearly in the same direction through a conduit wherein the speed is controlled by the speed of a roll assembly means. The tension and temperature are controlled by the gas flow rate, the diameter or cross-section of the conduit (which controls the gas velocity), and the length of the conduit. The gas may be introduced at one or more locations along the conduit. Pneumatic spinning allows for spinning speeds in excess of 5,000 meters per minute.
Tenacity is a key fiber property for industrial fibers. Tenacity is obtained by drawing quenched fibers in stages. This drawing in stages works well with cross flow at currently commercially available low speeds. An example of a known cross-flow quench and coupled spin-draw apparatus is shown in FIG. 1. In this apparatus, a melted polyamide is introduced at 10 to a spin pack 20. The polymer is extruded as undrawn filaments 30 from the spin pack, which has orifices designed to give the desired cross section. The filaments are quenched after they exit the capillary of the spin pack to cool the fibers by cross-flow cooling air at 40 in FIG. 1. These filaments are converged into a yarn 60 with application of a conventional finish lubricant at 50 and forwarded by a feed roll assembly 70. The yarn is then fed to a first draw roll pair 80 and then to a second draw roll pair 100. A hot tube 90, or draw assist, may be used to facilitate the second stage of the draw process. The yarn is relaxed at puller rolls 110 and 120. Roll 110 is also known as a relaxation roll; it can run at lower speeds than draw roll assembly 100 to control yarn shrinkage. Roll 120 is also known as a let-down roll relaxes the yarn tension to allow winding on at a lower tension than the yarn experiences in drawing. A guide 130 lays down the yarn on a yarn package 140, where it is wound up.
A known melt extrusion and coupled multi-stage drawing assembly using a cross-flow quench system is shown in FIG. 2. The assembly of FIG. 2 is similar to that of FIG. 1, but does not include a hot tube as FIG. 1 does, since the hot tube may damage the fiber. In FIG. 2, the draw is accomplished through rolls instead of a hot tube. In this apparatus, a melted polyamide is introduced at 200 to a spin pack 210. The polymer is extruded as undrawn filaments 220 from the spin pack, which has orifices designed to give the desired cross section. The filaments are quenched after they exit the capillary of the spin pack to cool the fibers by cross-flow cooling air at 230 in FIG. 2. These filaments are converged into a yarn bundle as shown at 250 with application of a conventional finish lubricant at 240 and forwarded by a feed roll assembly 260. The yarn is then fed to a first stage draw roll pair 270, and then to a second draw roll pair 275. An optional third draw roll assembly 280 may be used to further draw the fiber. The yarn is relaxed at relaxation roll 285. A guide 290 lays down the yarn on a yarn package 295 which is rotated by a winder chuck and wound up.
It is not possible to achieve higher spinning speeds in the cross-flow quench systems of FIGS. 1 and 2 through the use of cross-flow quench so as to increase productivity. The ability to draw a yarn decreases significantly with the use of cross-flow, which reduces ultimate yarn tenacity. Moreover, it is important that the produced polyamide yarn has properties at least as good as those obtainable at slower speeds. In particular, it is desirable to maintain the desired tenacity, elongation-to-break and uniformity of the produced yarn. Thus, there is a need in the art to provide methods and apparatus for high speed spinning of yarn while maintaining these properties.
Difficulties in the use of high spinning speeds are especially evident in colored or delustered nylon yarns. Such yarns are extruded from nylon polymers containing pigments, which provide a color palette of wide variety. Nylon yarn polymers are often delustered by the addition of titanium dioxide or zinc sulfide. Typically, the delustered and/or pigmented nylon cause problems for melt extrusion, partly due to differences in the melt flow behavior, microstructure development and heat loss properties compared to un-pigmented or non-delustered nylon. The presence of an increased level of filament breaks when using delustered or pigmented polymers is a long-standing problem. It is known that an attempt to increase extrusion speeds exacerbates the broken filament problem. Thus, it would be desirable in particular to provide a high speed spinning process that produces pigmented polyamide yarn without experiencing filament breaks.